The File Handler uses four data structures during file I/O operations:
|File Control Description (FCD)
|A 100-byte data area containing information about the file in use.
|A data area into which records are read and from which records are written.
|A data area which contains the name of the file in use, as recognized by the operating system.
|Key definition block
|Used by the File Handler to hold index key information during operations on indexed files.